1. Field of the Invention
This invention relates to a method for removal of mercury from a reducing gas. More particularly, this invention relates to a regenerative method for removal of mercury from a reducing gas containing hydrogen and/or CO, such as a coal-gasifier fuel gas, at temperatures in the range of about 25° C. to about 300° C.
2. Description of Related Art
Because of the health effects of mercury on fish-eating populations, there is a growing concern of mercury emissions from man-made sources, especially from coal-burning power plants. Mercury regulations currently proposed by the U.S Environmental Protection Agency (EPA) for coal-fired power plants also are likely to be applied to the next-generation gasification systems, such as Integrated Gasification Combined Cycle (IGCC) power plants.
The removal of mercury from high-pressure fuels/synthesis gases generated by gasification processes is presently carried out using activated carbon-based processes at low temperature. However, for IGCC systems, such low-temperature processes result in severe energy penalties and reduced efficiencies because the pressurized fuels/synthesis gases produced by the system have to be reheated to the operating temperature (greater than about 300° C.) of the gas turbines employed for electricity generation. Consequently, current focus is on developing sorbents for removal of mercury from fuels/synthesis gases having temperatures in the range of about 150° C. to about 300° C. at pressures in the range of about 300 psi to about 1000 psi.
Development of a sorbent-based process for IGGC systems, however, is a major challenge and currently no proven technologies exist for the removal of trace levels of mercury from high-temperature fuels/synthesis gases. Activated carbon-based sorbents remove mercury primarily by a physisorption mechanism, and they are not effective at higher temperatures. In addition, exposure of impregnated sorbents to gaseous streams at high temperature (greater than about 120° C.) has been shown to release some of the active sulfur component in the gas stream, making the sorbent ineffective.
Another substantial issue is the presence of highly reactive hydrogen in the fuels/synthesis gases which can alter the chemical characteristics of an otherwise active sorbent, making it ineffective for mercury removal. In addition, the reaction or interaction of mercury with the active component in the sorbent may not be favorable in the presence of hydrogen. For example, the stability of HgS and HgSe mercury species in reducing gases has been shown to be very low at temperatures greater than about 300° C.
Thus, there is a need for a process that can remove mercury from solid fuel gasifier fuels/synthesis gases at high temperature. There is also a substantial interest in reducing emissions of other heavy elements, such as arsenic, selenium, and cadmium along with the mercury due to the environmental impact of these emissions. A process which can remove these heavy elements in a regenerative manner will have the added benefit of using the expensive sorbent over many cycles.
CuS-based sorbents have been widely used in the removal of mercury from oil and natural gas. CuS reacts with mercury in the gas stream and forms a stable HgS product in accordance with the following equation:2CuS+Hg(g)→Cu2S+HgS  (1)
U.S. Pat. No. 4,094,777 discloses a process for capturing mercury from a gas or a liquid using a supported CuS sorbent. The solid dispersant or support is selected from the group formed by silica, alumina, and others and the amount of copper in the sorbent is in the range 2 to 65 wt %.
Studies have focused on the development of sorbents based on metals, their oxides, sulfides, and selenides. Studies of the mercury removal potential of activated carbon and iron oxide from fuel gases over the temperature range of about 80° C. to about 175° C. indicate that mercury removal activity is accelerated by the presence of H2S. The studies suggest that active sulfur sites are generated on the sorbent surface in the presence of H2S, which in turn interact with gas-phase mercury.
U.S. Pat. No. 4,902,662 discloses a method for preparing supported CuS sorbents. The method comprises incorporating a copper compound with a solid inorganic carrier, optionally roasting this product in air to generate copper oxide, incorporating in the oxide at least one organic polysulfide, and thermally treating the resultant mass to produce copper sulfide.
U.S. Pat. No. 6,007,706 discloses a process for removing sulfur together with other contaminants such as mercury and arsenic from fluids. This process utilizes a bed containing a copper compound that is converted into copper sulfide by the sulfur component in the fluid, which copper sulfide subsequently removes the mercury and arsenic.
U.S. Pat. No.4,593,148 discloses a method for capturing arsine and hydrogen sulfide by making use of a bed of copper oxide and zinc oxide. U.S. Pat. No. 4,605,812 discloses that arsines can be removed from inert gases by contacting the streams with copper (II) chromite catalyst. U.S. Pat. No. 3,812,652 discloses that copper oxide used to remove arsenic from hydrocarbon streams can be regenerated by contacting the copper oxide with a stream of molecular oxygen in the range 150-700° F.
U.S. Pat. No. 4,853,110 discloses a method for removing arsenic and/or selenium from shale oil by reaction with a metal oxide and/or metal sulfide, where the metal is selected from a group consisting of molybdenum, nickel, and cobalt.
Although the use of supported CuS sorbents for capture of mercury and/or arsenic from certain fluid streams is known, the removal of these impurities from a gas stream containing high concentrations of H2 and/or CO at temperatures higher than 150° C. is not known. Under such conditions, CuS-based sorbents are not effective. Thermodynamically, CuS is reduced in the presence of H2 and its reduced form, Cu2S, is ineffective for mercury capture. In addition, sulfur from the sorbent is released in the form of H2S in accordance with the following reaction:2CuS+H2(g)→Cu2S+H2S(g)  (2)